The present invention relates to improvements in mechanisms for an improved multifilament extrusion process and more particularly to an improved heating mechanism for heating and stretching plastic raffia or filaments or similar material.
In mechanisms for producing extended plastic filaments used in the production of rope or for like purposes, the material is extruded and stretched. In materials such as polypropylene in filament or raffia state, the filaments are heated to a predetermined temperature while applying an axial force. The filaments may be in circular or flat form, and in the flat form provide tape yarns which are employed for textile or rope uses. The yarns or filaments must be stretched with the application of an axial tension, and they are pulled a predetermined amount at a predetermined temperature to a point where the material forms a hair-like structure. This grain direction orientation greatly strengthens and conditions the material for its end use. It is known that molecular orientation of all plastic materials from which yarns may be obtained by means of extrusion increases their toughness as a function of the stretching ratio.
In this operation a critical portion is the application of heat. The filaments are usually provided directly from an extruder or from rolls on which they have been wound when previously extruded and are supplied in multistrands through a heat source for stretching and elongating. The application of heat must be controllable and applied rapidly and uniformly in order to obtain relatively high production speeds. In devices heretofore available because of difficulties encountered in using high speed heated air blown countercurrently to the stretched filaments, it has been impossible to obtain satisfactory production speeds, and speeds limited to 100 meters per minute have been common, higher speeds causing strong weakening and size reduction of the filaments and finally their breaking.
It is an object of the present invention to provide an improved stretching and heating mechanism for the manufacture of plastic such as polypropylene in the filament or raffia state capable of higher operating speed on the order of 250 - 300 meters per minute. On an experimental laboratory equipment, speeds up to 700 meters per minute were also obtained.
A further object of the invention is to provide a mechanism for producing stretched plastic filaments which is capable of more uniformly and more satisfactorily heating the filaments during the stretching stage.
A feature of the invention is drawing the filaments over antifriction high temperature resistant material to heat the filaments by conductive heat while simultaneously subjecting the filaments to radiant heat from a heater opposite the surface while applying the tensile axial stretching force to the filaments to draw them over the arcuate surface.
Other features, objects and advantages will become more apparent, as will equivalent structures which are intended to be covered, in the teaching of the principles of the invention in connection with the disclosure of a preferred embodiment thereof in the specification, claims, and drawings, in which: